A spark plug for providing ignition in an internal combustion engine, such as an automobile engine, generally includes a tubular metallic shell and a tubular insulator disposed in a bore of the metallic shell. A center electrode is disposed at the forward side of an axial bore of the insulator, and a metal terminal is disposed at the rear side of the axial bore. A ground electrode has one end that is joined to the forward end of the metallic shell and another end facing the center electrode and forms a spark discharge gap in cooperation with the center electrode.
In recent years, there has been developed a technique for increasing driving distance with less fuel through improvement of output by use of a supercharger. In such an internal combustion engine, temperature within a combustion chamber tends to increase. As a result, the working environment of a spark plug is becoming more and more severe. When the spark plug is subjected to repeated temperature rise and drop (hereinafter, may be referred to as heating and cooling cycles) within the combustion chamber, the center electrode may become loose and have play in the axial bore of the insulator. Although the center electrode is fixed within the axial bore of the insulator by means of a seal material, as a result of subjection to repeated heating and cooling cycles, fixing strength between the center electrode and the seal material is apt to deteriorate.
Regarding such a problem, in order to enhance the strength of fixing the center electrode in the insulator, for example, claim 1 in Japanese Patent Application Laid-Open (kokai) No. 2010-267425 provides “a spark plug . . . characterized in that the head of the center electrode has grooves formed along its outer circumference.”
Also, in order to ensure sufficient strength and impact resistance by means of a glass seal material, claim 1 in Japanese Patent No. 3497009 provides “a spark plug . . . characterized in that the diameter d0 of the large-diameter portion of the axial bore, the diameter d1 of the flange portion, and the diameter d2 of the head portion satisfy a relational expression 0≦d1−d2≦1 mm and that the flange-head length h from the forward end of the flange portion to the rear end of the head portion falls in a range of 2.0≦h≦3.0 mm and in a range of ±25% of (d0−d1)×5.”
An object of the present invention is to provide a spark plug having good adhesion between a center electrode and a seal material.